The technology of the present disclosure relates generally to distributed antenna systems (DASs) that include distribution of reference signals, and particularly to providing redundancy for reference signal generation used to synchronize frequency conversion circuits in the DAS.
Wireless communication is rapidly growing, with ever-increasing demands for high-speed mobile data communication. As an example, local area wireless services (e.g., so-called “wireless fidelity” or “WiFi” systems) and wide area wireless services are being deployed in many different types of areas (e.g., coffee shops, airports, libraries, etc.). Distributed communications or antenna systems communicate with wireless devices called “clients,” “client devices,” or “wireless client devices,” which must reside within the wireless range or “cell coverage area” in order to communicate with an access point device. Distributed antenna systems are particularly useful to be deployed inside buildings or other indoor environments where client devices may not otherwise be able to effectively receive radio-frequency (RF) signals from a source, such as a base station for example. Example applications where distributed antenna systems can be used to provide or enhance coverage for wireless services include public safety, cellular telephony, wireless local access networks (LANs), location tracking, and medical telemetry inside buildings and over campuses.
One approach to deploying a distributed antenna system involves the use of RF antenna coverage areas, also referred to as “antenna coverage areas.” Antenna coverage areas can be formed by remotely distributed antenna units, also referred to as remote units (RUs). The remote units each contain or are configured to couple to one or more antennas configured to support the desired frequency(ies) or polarization to provide the antenna coverage areas. Antenna coverage areas can have a radius in the range from a few meters up to twenty meters as an example. Combining a number of remote units creates an array of antenna coverage areas. Because the antenna coverage areas each cover small areas, there typically may be only a few users (clients) per antenna coverage area. This arrangement generates a uniform high quality signal enabling high throughput supporting the required capacity for the wireless system users.
As an example, FIG. 1 illustrates distribution of communications services to coverage areas 10(1)-10(N) of a DAS 12, wherein ‘N’ is the number of coverage areas. The coverage areas 10(1)-10(N) may be remotely located. In this regard, the remote coverage areas 10 are created by and centered on remote antenna units 14(1)-14(N) connected to a head-end equipment 16 (e.g., a head-end controller or head-end unit). The head-end equipment 16 may be communicatively coupled to a base station 18. In this regard, the head-end equipment 16 receives downlink communications signals 20D from the cellular base station 18 to be distributed to the remote antenna units 14(1)-14(N). The remote antenna units 14(1)-14(N) are configured to receive downlink communications signals 20D from the head-end equipment 16 over a communications medium 22 to be distributed to the respective coverage areas 10(1)-10(N) of the remote antenna units 14(1)-14(N). Each remote antenna unit 14(1)-14(N) may include an RF transmitter/receiver (not shown) and a respective antenna 24(1)-24(N) operably connected to the RF transmitter/receiver to wirelessly distribute the cellular services to client devices 26 within their respective coverage areas 10(1)-10(N). The remote antenna units 14(1)-14(N) are also configured to receive uplink communications signals 20U from the client devices 26 in their respective coverage areas 10(1)-10(N) to be distributed to the cellular base station 18. The size of a given coverage area 10(1)-10(N) is determined by the amount of RF power transmitted by the respective remote antenna unit 14(1)-14(N), the receiver sensitivity, antenna gain and the RF environment, as well as by the RF transmitter/receiver sensitivity of the cellular client device 26. Cellular client devices 26 usually have a fixed RF receiver sensitivity, so that the above-mentioned properties of the remote antenna units 14(1)-14(N) mainly determine the size of their respective remote coverage areas 10(1)-10(N).
As an example, it may be desired to synchronize the downlink communications signals 20D distributed to the remote antenna units 14(1)-14(N). For example, the downlink communications signals 20D may be Ethernet signals that need to be synchronized among remote antenna units 14(1)-14(N). As another example, communications medium 22 in the DAS 12 in FIG. 1 may have a lower frequency handling rating than the frequency of the downlink communications signals 20D and/or uplink communications signals 20U. In this regard, the DAS 12 in FIG. 1 may provide for frequency conversion of the downlink and uplink communications signals 20D, 20U before being provided on the communications medium 22 to be distributed. The downlink and uplink communications signals 20D, 20U can be frequency shifted to an intermediate frequency within the frequency rating of the communications medium 22.
In this regard, FIG. 2 is a schematic diagram of frequency conversion components that can be provided in the DAS 12 in FIG. 1 to frequency convert the downlink communications signals 20D in the head-end equipment 16 before being provided on the communications medium 22 to be distributed to the remote antenna units 14(1)-14(N). Only the components involved in frequency conversion of the downlink communications signals 20D are shown in FIG. 2 for illustration purposes, but other components can be provided to frequency convert the uplink communications signals 20U. As illustrated in FIG. 2, a master oscillator 28 is provided in the head-end equipment 16 to generate a reference signal 30. The reference signal 30 is provided to a local oscillator 32 in the head-end equipment 16 to control the generation of a mixing signal 34 by the local oscillator 32. A mixer 36 mixes the downlink communications signal 20D with the mixing signal 34 to down convert the frequency of the downlink communications signal 20D to provide an intermediate frequency (IF) downlink communications signal 20D(IF). For example, if the frequency of the downlink communications signal 20D is F1 and the frequency of the mixing signal 34 is F2, the frequency of the IF downlink communications signal 20D(IF) will be F1-F2 with appropriate filtering to filter off the sideband F1+F2. The IF downlink communications signal 20D(IF) is communicated over the communications medium 22 to the remote antenna units 14(1)-14(N).
With continuing reference to FIG. 2, using remote antenna unit 14(1) as an example, a mixer 38(1) is provided in the remote antenna unit 14(1) to up convert the frequency of the IF downlink communications signal 20D(IF) back to the original frequency of the downlink communications signal 20D. This is so the remote antenna unit 14(1) can distribute the downlink communications signal 20D in its original frequency to client devices 26 (in the coverage area 10(1) of the remote antenna unit 14(1)) (see FIG. 1). In this regard, a local oscillator 40 is provided that is configured to generate a mixing signal 42 of appropriate frequency to be mixed with the IF downlink communications signal 20D(IF) to frequency up convert and recover the downlink communications signal 20D as recovered downlink communications signal 20D(R). However, it may be important for the local oscillator 40 to generate the mixing signal 42 that is synchronized with the mixing signal 34 in the head-end equipment 16 so that the frequency and/or phase of the recovered downlink communications signal 20D(R) is synchronized to the frequency and/or phase of the downlink communications signal 20D. Thus, as illustrated in FIG. 2, the reference signal 30 generated by the master oscillator 28 is also provided over the communications medium 22 to the remote antenna units 14(1)-14(N). The reference signal 30 is used to control the generation of a mixing signal 42(1) generated by the local oscillator 40(1) in the remote antenna unit 14(1) so that the mixing signal 42(1) is synchronized with the mixing signal 34. In the example in FIG. 2, the reference signal 30 is provided to a phase-locked loop (PLL) circuit 44(1) that is configured to generate a PLL control signal 46(1) to control the local oscillator 40(1) to generate the mixing signal 42(1) in frequency and phase synchronization with the mixing signal 34. In this manner, the mixer 38(1) can generate the recovered downlink communications signal 20D(R) in frequency and phase synchronization with the original downlink communications signal 20D.
While the master oscillator 28 in the head-end equipment 16 in FIG. 2 provides for the ability to generate a reference signal 30 that can be used and distributed for synchronization purposes, it also provides for a single failure point in the DAS 12. If the master oscillator 28 fails to generate the reference signal 30, the components in the DAS 12 that receive and use the reference signal 30 for synchronization purposes may also fail as a result.